The present invention relates generally to gas turbine engines, and, more specifically, to combustors therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases. Energy is extracted from the combustion gases in turbine stages which power the compressor and produce useful work such as powering a fan disposed upstream of the compressor.
A turbofan engine operates over various power levels at idle, takeoff, and cruise and is designed to maximize efficiency for reducing fuel consumption and undesirable exhaust emissions.
Typical exhaust emissions include smoke, unburned hydrocarbons, carbon monoxide (CO), and nitrogen oxides (NOx). Due to the complex nature of combustion, and due to the limited space available in typical combustors, many compromises must be made in combustor design for achieving acceptable performance for particular engine designs and power ratings.
The prior art is quite crowded with various types of gas turbine engine combustors with myriad configurations thereof for maximizing performance or minimizing emissions with practical combustor life or durability.
A typical single annular combustor includes radially outer and inner combustion liners joined at forward ends to an annular dome and spaced apart at aft ends to define an annular outlet for discharging combustion gases to the turbines. The dome includes a plurality of circumferentially spaced apart carburetors each including a fuel injector and cooperating swirler or swirl cup. Various designs are found for the feel injectors and the swirlers.
A typical swirler supports a fuel injector tip and mixes with the fuel therefrom compressed air typically swirled in counter-rotating streams therearound. Air swirling is typically effected by rows of inclined swirl vanes or inclined air holes formed in the body of the swirl cup.
A fuel and air mixture discharged from each carburetor into the combustor undergoes combustion therein which liberates substantial heat. The various components of the combustor are protected from this beat by diverting a portion of the pressurized air for use as cooling air.
A typical combustion liner is formed of annular panels axially joined together at corresponding film cooling nuggets which create a film of cooling air along the inner surface of the combustor bounding the combustion gases therein.
Air is also channeled through the dome for cooling thereof. The dome is additionally cooled by providing corresponding heat shields or baffles around each of the swirlers which pass through the dome to provide a barrier against the hot combustion gases. The heat shields are cooled by holes in the dome which impinge air thereon. The spent impingement air then flows along the heat shields for discharge into the combustor.
Each swirler also includes an outlet in the form of a flare cone at its aft end which protrudes through a corresponding heat shield. The included flare angle of the cone controls the corresponding spray angle of the fuel and air mixture from each of the carburetors.
Since the flare cones are directly exposed to the combustion gases, they are subject to heating therefrom. To prevent burning of the flare cone, air may be channeled between the aft side of the heat shield and the front side of the flare cone for providing convection cooling thereof, with the spent air being discharged into the combustor. Combustors including this type of flare cone cooling have been sold in this country for many years. However, this type of flare cone cooling has been found to be a source of hydrocarbon and CO emissions, which are undesirable under more stringent emission requirements.
Another combustor enjoying many years of successful commercial use in this country avoids this emission problem by eliminating air cooling of the flare cone and instead integrating the heat shield with the aft end of the flare cone in one casting. However, the interface between the flare cone and the integral heat shield provides a structural discontinuity for the different functions thereof, and the integrated component is subject to substantial temperature gradient from relatively cool in the flare cone to relatively hot at the aft end of the heat shield exposed to the combustion gases. The large temperature gradient and geometric discontinuity create substantial thermal stress during operation which correspondingly reduces the effective life of the components.
Other types of combustors enjoying years of successful commercial use in this country use an acute flare cone angle of up to about 90.degree. to maintain a narrow angle fuel spray increasing the spacing from the heat shields. The intent of the narrow spray angle is to direct the unburned fuel away from the liner walls to minimize unburned hydrocarbon and CO emissions. In addition, the narrow spray draws hot combustion gas from the downstream area into close contact with the beat shield, increasing the above mentioned temperature gradient However, this solution is not practical on relatively small combustors since altitude restarting is severely compromised.
Accordingly, it is desired to provide an improved combustor having reduced exhaust emissions and suitable durability.